Process control is a long-established art which plays a major role in managing industrial plants and processes. In this art, process transmitters have been used to monitor process variables. Having evolved from the earliest measurement devices such as barometers and thermometers, the process transmitter has traditionally received a great deal of technological attention to improve performance due to the need for accurate process measurement. Since the accuracy of every measurement made in a process control loop is directly dependent upon the accuracy of the particular process transmitter or instrument which closes the loop, the process transmitter plays a particularly sensitive role in industrial process control systems.
Beginning in the 1950s, electrical and electronic process control loops were a natural development from prior electromechanical control systems. The general problem of electronic process control is to convert a physical variable to an electrical signal, and to subsequently transmit that signal to a recorder and/or other control equipment which may be located some distance away from the physical variable. Early types of process control loops to accomplish this goal were "four-wire" systems, and were configured such that operating power was supplied through two of the four wires and a process signal was transmitted through the other two wires. The four-wire system requires the use of amplifiers or other signal conditioning equipment at the point of measurement in order to supply an accurate signal representative of the physical variable since the process signal is generally very low. See, e.g., U.S. Pat. No. 3,680,384, of Grindheim. Prior four-wire transmitter systems thus required separate power supply lines, and voltage power supplies.
After the four-wire transmitter was developed, it became apparent that the advantages of using the same two wires for power supply and information transmission would greatly improve the process control art. The "two-wire" transmitter was then developed and operates today in a control loop in conjunction with an external power supply, a pair of wires from the supply, and a transmitter connected serially between the wires. As used herein, the term "two-wire" is construed broadly to mean two conductors. Thus, the term "two-wire" includes actual wires, twisted pairs, coaxial cables, and other pairs of conductors.
During operation of such a two-wire transmitter loop, the transmitter energizes a sensor element and receives informational signals from the sensor element. The information is transmitted on the pair of wires by varying the current in the current loop. Thus the transmitter acts as a variable current sink, and the amount of current which it sinks is representative of the information from the sensor. Such prior two-wire transmitter loops have generally been analog in nature, and the industry standard which has developed for two-wire transmitters is a 4 to 20 milliamp loop, with a variable loop supply voltage having a maximum output of 24 volts DC. With such a low voltage supply, two-wire transmitter loops are particularly suited for use in hazardous environments. See, e.g., U.S. Pat. No. 4,242,665, of Mate.
More advanced prior two-wire transmitter control loops exhibit high-level data communication between two-wire transmitters and various receiving elements, for example controllers and communication devices. The concept of digital communication in 4 to 20 milliamp control systems is known for use in the more complicated 4 to 20 milliamp loops having both digital and analog components. Transmitters suitable for such purposes are usually called "smart" transmitters because they are more accurate and have operating parameters which may be remotely controlled.
As technology has progressed over the years, low powered microprocessors have made it possible to transport smart field transmitters into the digital signal processing environment. Furthermore, digital microprocessors make it possible to improve the accuracy of smart two-wire transmitters. Since digital microprocessors are typically used in the transmitters, a clock or oscillator circuit is required to provide a system clock to the central processing unit (CPU) which effectively runs the digital microprocessor and the transmitter's digital components.
The trend in two-wire transmitter loops both in the smart, microprocessor-based transmitter area and the traditional analog transmitter area, has been to reduce the power requirements for components which are used in the loop. This need has arisen since the amount of power which a two-wire transmitter may draw from a current loop to use for its operation is severely limited. With a nominal 10-volt supply, at the bottom end of operation only about 40 milliwatts is available to power any instrumentation in the loop. Thus with large power demands on the loop, two-wire control systems may be limited to a few low power industrial control applications. This aspect of industrial controls competes with the general desire to design instrumentation into the loop to simplify loop operation and installation, and to provide intrinsic safety in a low power process control environment. This long-felt need has not adequately been met by process control loops which have the aforementioned inherent power budget problems.
System components, for example, the aforementioned system clock, must therefore be low power components which do not unduly load the transmitter. It is thus important to provide oscillator components in a two-wire transmitter and control loop with improved power consumption efficiencies to enlarge the scope of versatility of such transmitters in low power two-wire systems.